1. Field of the Invention
The present invention relates to a color filter method and apparatus for manufacturing a color filter by forming a large number of pixels colored in a plurality of kinds of colors on a transparent substrate, an ink-jet device, a color filter, a display device, and an apparatus having the display device.
2. Description of the Related Art
With recent advances in personal computers, especially portable personal computers, the demand tends to arise for liquid crystal displays, especially color liquid crystal displays. However, in order to further popularize the use of liquid crystal displays, a reduction in cost must be achieved. Especially, it is required to reduce the cost of a color filter which occupies a large proportion of the total cost. Various methods have been tried to satisfy the required characteristics of color filters while meeting the above requirements. However, any method capable of satisfying all the requirements has not been established. The respective methods will be described below.
The first method is a dyeing method. In the dyeing method, a water-soluble polymer material as a dyeable material is applied to a glass substrate, and the coating is patterned into a desired shape by a photolithographic process. The obtained pattern is dipped in a dye bath to obtain a colored pattern. This process is repeated three times to form R, G, and B color filter layers.
The second method is a pigment dispersion method, which is currently replacing the dyeing method. In this method, a pigment-dispersed photosensitive resin layer is formed on a substrate and patterned into a single-color pattern. This process is repeated three times to obtain R, G, and B color filter layers.
The third method is an electrodeposition method. In this method, a transparent electrode is patterned on a substrate, and the resultant structure is dipped in an electrodeposition coating fluid containing a pigment, a resin, an electrolyte, and the like to be colored in the first color by electrodeposition. This process is repeated three times to form R, G, and B coatings. Thereafter, the resin layers are thermoset to form colored layers.
The fourth method is a print method. In this method, a pigment is dispersed in a thermosetting resin, and a print operation is performed three times to form R, G, and B coatings separately, and thermosetting the resins, thereby forming colored layers. In either of the above methods, a protective layer is generally formed on the colored layers.
The point common to these methods is that the same process must be repeated three times to obtain layers colored in three colors, i.e., R, G, and B. This causes an increase in cost. In addition, as the number of processes increases, the yield decreases. In the electrodeposition method, limitations are imposed on pattern shapes which can be formed. For this reason, with the existing techniques, this method cannot be applied to a TFT type color liquid display. In the print method, a pattern with a fine pitch cannot be formed because of poor resolution and poor evenness.
In order to eliminate these drawbacks, methods of manufacturing color filters by an ink-jet system are disclosed in Japanese Patent Laid-Open Nos. 59-75205, 63-235901, 63-294503, and 1-217320.
These references disclose, for example, a method of forming a color filter by an ink-jet method, in which a light-shielding film is formed on a transparent substrate to have opening portions with predetermined regularity, and an ink is discharged from an ink-jet head onto the opening portions to color the substrate.
In general, it takes much time and cost to develop an ink-jet head. Even if a custom ink-jet head is developed for a color filter manufacturing apparatus, since the production is smaller than that of ink-jet heads generally used for printers and the like, the cost of the custom ink-jet head is very high. As a result, the cost of the manufacturing apparatus becomes high, and hence the cost of a color filter increases.
Assume that ink-jet heads generally used for printers are applied to the manufacture of color filters. The resolution of most of the existing printer ink-jet heads is 300 dpi, 360 dpi, 400 dpi, 600 dpi, or 720 dpi. The pixel pitch of a color filter of a 9.4-inch VGA type is 300 μm (corresponding to 84.7 dpi). The pixel pitch of color filters of a 10.4-inch XGA type and a 12.9-inch EWS type is 207 μm (corresponding to 122.7 dpi). The pixel pitch of color filters of a 12.1-inch XGA type, a 13.8-inch EWS, and a 15.5-inch EWS type is 240 μm (corresponding to 105.8 dpi). There is no combination of an ink-jet head and a color filter whose resolution and pixel pitch match with each other or are an integral multiple. Considering other combinations, there is hardly any combination of an ink-jet head and a color filter whose resolution and pixel pitch match with each other or are an integral multiple.
When, therefore, an ink-jet head used for a general ink-jet printer is to be applied to a color filter manufacturing apparatus, the present applicant has studied a technique of matching the resolution of the ink-jet head with the pixel pitch of a color filter by positioning the ink-jet head obliquely with respect to a color filter substrate.
The present applicant has also studied a technique of mounting the ink-jet head on the manufacturing apparatus such that the angle of the ink-jet head can be changed with respect to the a color filter substrate to allow the single ink-jet head to cope with the pixel pitches of various types of color filters.
When, however, the ink-jet head is mounted obliquely with respect to a color filter, a coloring operation is performed by relatively scanning the ink-jet head and the color filter, a relative scanning operation is required in excess of an amount corresponding to the oblique positioning of the ink-jet head. This increases the coloring time required for one substrate. Especially when an ink-jet head shorter than the effective pixel area of a color filter is used, since a relative scanning operation must be performed a plurality of numbers of times, a longer coloring time is required. With an increase in coloring time per substrate, the production of color filters per unit time decreases, resulting in an increase in the cost of a color filter.
In addition, when an ink-jet head to be used is caused to stand, an ink in discharging nozzles of the ink-jet head increases its viscosity or solidifies upon contacting air. As a result, a discharge failure, i.e., inability to discharge the ink, or twisting, i.e., tilting of an ink discharging direction, may occur. For this reason, the nozzle surfaces are covered with cap members when the ink-jet head is not used. Even with the cap members, in an initial period of a discharging operation, first some inks to be discharged from one nozzle may not be discharged, or twisting may occur because of the influence of the ink whose viscosity has increased. If the ink is discharged onto a recording medium such as a paper sheet in this state, part of a character may be omitted or become excessively bright or dark, resulting in a deterioration in print quality. In order to prevent such a phenomenon, a receiving portion for receiving the pre-discharged ink is formed in a cap portion or the like, and several inks are preliminarily discharged (pre-discharging operation) onto the receiving portion before an actual printing operation is performed. This operation itself is generally performed in an ink-jet printer, and hence is not a special operation.
When an ink-jet head is applied to a color filter manufacturing apparatus, unlike in a general printer, a coloring operation is performed by discharging an ink from the ink-jet head onto opening portions having predetermined regularity, as described above. Owing to this method, the color filter manufacturing apparatus demands an ink landing accuracy 10 times higher than that in the general printer. For this reason, a pre-discharging operation becomes more important, and it is preferable that a pre-discharging operation always be performed before a color pattern is formed by discharging an ink onto a glass substrate.
When the manufacturing apparatus uses a long head which can color an entire glass substrate with one scanning operation after this pre-discharging operation, the following problem is not posed. When, however, the apparatus uses a short head which colors a glass substrate with a plurality of numbers of times of scanning operations, since the position of the head relative to the substrate is changed between the scanning operations, an idling time during which the discharging operation of the head is stopped is required. If this time increases, a discharge failure or twisting may occur in discharging the ink in the next scanning operation.
In coloring the entire substrate surface with a plurality of numbers of times of scanning operations, the same nozzles are not necessarily used for each scanning operation owing to the relationship between the number of discharging nozzles which can used in one scanning operation, and the number of pixels constituting a color filter. For this reason, nozzles which have not been used in the previous scanning operation may be used.
As is apparent, since the ink in the nozzles which have not been used in the previous scanning operation contact air without being used for a long period of time as compared with the ink in the remaining nozzles, the ink in the nozzles which have not been used may increase its viscosity or solidify. For this reason, when the ink is to be discharged from the nozzles which have not been used, a discharge failure, i.e., inability to discharge the ink, or twisting, i.e., tilting of an ink discharging direction, may occur. As a result, a defective color filter may be manufactured.
In order to prevent this, the head may be relatively moved to an ink-receiving portion for a pre-discharging operation, which is formed in the cap portion or the like to perform a pre-discharging operation every time a scanning operation is performed. With this operation, however, the time required to completely color one substrate increases, and the production per unit time decreases, resulting in an increase in the cost of a color filter. That is, the present applicant found this method undesirable.
In addition to the above problem, the following problem is also posed.
Prior to a detailed description of this problem, the following case studied by the present applicant will be described below with reference to FIG. 1. In applying an ink-jet head used for a general ink-jet printer to a color filter manufacturing apparatus, the ink-jet head is mounted obliquely with respect to a color filter substrate to match the resolution of the ink-jet head with the pixel pitch of a color filter.
FIG. 1 is a plan view showing how the pixels of a color filter are colored by an ink-jet head. With regard to the ink-jet head, only the position of a nozzle array is shown in FIG. 1. In this state, of a predetermined pattern, a portion to be colored in red is being colored. Note that the letters R, G, and B in the respective pixels in FIG. 1 indicate that the respective pixels are to be colored in red (R), green (G), and blue (B).
Reference numeral 1013 denotes a nozzle array formed on the ink-jet head. An ink is discharged from this nozzle array to form ink dots on the substrate. Reference numeral 1014 denotes each pixel of a color filter. On these pixels on the substrate, ink dots are formed.
In the case shown in FIG. 1, since the pixel pitch of the color filter does not coincide with the nozzle pitch of the ink-jet head, the head is tilted to make the positions of every three pixels of the same color in the Y direction coincide with the positions of the ink discharged from every five nozzles. Ink dots are then formed in the pixels 1014 while the ink-jet head is relatively moved in the X direction in FIG. 1, thereby coloring the pixels. This operation is performed using ink-jet heads for discharging red, green, and blue inks to manufacture a color filter. In this operation, in the ink-jet head for coloring red pixels, which is shown in FIG. 1, the second, seventh, and twelfth nozzles are used to discharge the ink, but the remaining nozzles are not used.
In this case, as this ink-jet head, a general ink-jet head having a nozzle pitch of 360 dpi (70.5 μm) is used. As the color filter, a filter having a pixel pitch of 100 μm is used.
This ink-jet head has the following characteristics:
(1) When a nozzle clogs with a solid matter in an ink, a normal discharging operation may not be performed. In this case, if a nozzle in use does not properly discharge an ink, the entire ink-jet head is replaced.
(2) In an ink-jet system using a thermal phenomenon, in particular, when a predetermined number of inks are discharged, the nozzle becomes incapable of properly discharging an ink because of scorching of the ink or the like. That is, the service life of each nozzle depends on the number of inks discharged. For this reason, an ink-jet head must be periodically replaced.
(3) In the ink-jet system using the thermal phenomenon, since a heater provided for each nozzle which is discharging an ink generates heat, repetition of a discharging operation will raise the temperature near the nozzle. The amount of ink discharged depends on temperatures. For this reason, ink dots gradually change in size, and the ink-jet head expands, resulting in deformation such as a change in nozzle pitch.
As described above, in a color filter manufacturing apparatus, if only specific nozzles are used, the following problems are posed:
(1) When a given nozzle becomes incapable of properly discharging an ink, the entire ink-jet head must be replaced, although the remaining nozzles can be used.
(2) In the ink-jet system using the thermal phenomenon, in particular, since the service life of each nozzle depends on the number of inks discharged, head replacement must be periodically performed. In this replacement, the overall service life of the ink-jet head coincides with the service life of some nozzles which have been used, although the majority of the remaining nozzles can be used.
(3) In the ink-jet system using the thermal phenomenon, in particular, as a discharging operation is performed, the temperature near each nozzle in use rises, and the amount of ink discharged varies. For this reason, the size and color density of ink dots formed on the substrate vary, adversely affecting the uniformity of a color filter.
The following problem is also posed. An impurity may elute from a coloring material contained in an ink used for a color filter into a liquid crystal layer to cause a deterioration in display quality. For this reason, this ink must be purified to remove such an impurity. Consequently, the ink becomes expensive. In order to achieve a reduction in cost, a demand has arisen for a reduction in the amount of ink used.